• 한국자동차공학회논문집
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• 제9권3호
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• pp.181-189
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• 2001

For the Tailor Welded Blank sheet used fur automobile body panel, the characteristics of fatigue crack propagation behavior were studied. The thickness of specimen was machined to be same (0.9+0.9mm) and different (0.9+2.0mm). As a base test, mechanical properties around welding zone were examined. The results indicated that there were no significant decreases in mechanical properties, but hardness around welding bead is 2.3 times greater than base material. The crack propagation rate was noticeably decreased around welding line and rapidly increased as it passed by welding line. Reviewing the shape of the crack propagation, crack width around welding line was wide around the welding zone due to retardation of crack growth, but it became narrow passing welding line due to decreased toughness. Elasto-Plastic analysis was performed by finite element analysis fur explaining the test results.

• 한국자동차공학회논문집
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• 제11권1호
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• pp.143-150
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• 2003

Experimental research has been carried out to investigate the characteristics of the fatigue crack initiation and propagation behavior of Tailor Welded Blank(TB) sheet used for vehicle body. We used three types of specimens which were machined of the same base metal: one is 1.4mm thick, another is 1.6mm thick, and the third(TB specimen) is laser-welded of two specimens(1.4mm and 1.6mm thick ones). The results of tensile and hardness test indicate that the yield strength of the TB specimen is the highest, and the hardness around welding bead is higher than that of base metal. Fatigue strength and fatigue limit of the TB specimen are much superior to those of the base metal up to $10^6$ cycles. The fatigue crack propagation of the heat-affected zone of the TB specimen is slower than that of the base metal. Welding bead has the fastest crack Propagation in the low stress intensity factor range$(\DeltaK)$ region, but the slowest in the high $\DeltaK$ region. The fatigue propagation characteristic of the TB specimen is relatively stable in comparison with that of the base metal in the high ${\Delta}K$ region around over $28MPa\sqrt{m}$.

• 한국레이저가공학회지
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• 제7권2호
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• pp.1-10
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• 2004

This work was attempted to join SCP sheet and STS304 sheet by using Nd:YAC laser beam. SCP sheet has good formability and low cost, while STS304 has excellent corrosion resistance and mechanical properties in high temp. In this experiment, butt joint type was used to develop the tailored blank welding for dissimilar steel. Sheets which have different thermal properties. Computer simulation was conducted to obtain the off-set position for efficient welding by considering laser power, scanning speed, focal length and basic properties. The result showed that the optimum thermal distribution was obtained when the laser beam was irradiated at $0.05{\sim}0.1$ mm off-set toward the SCP sheet side. The experiment was conducted based on the result of computer simulation to show the same optimum conditions. Optimum conditions were 3KW in laser beam power, 6m/min in scanning speed, -0.5mm in focal position, 0.1mm off-set toward SCP. Microhardness test, tensile test, bulge test, optical microscopy, EDS, and XRD were performed to observe the microstructure around fusion zone and to evaluate the mechanical properties of optimum conditions, The weld zone had high microhardness values by the formation of the martensitic structure. Tensile test measured the strength of welded region by vertical to strain direction and the elongation of welded region by parallel to strain direction. Bulge test showed $52\%$ formability of the original materials. Bead shape, grain size, and martensitic structure were observed by the optical microscopy in the weld zone. Detailed results of EDS, XRD confirmed that the welded region was connected of martensitic structure.

• 한국공작기계학회:학술대회논문집
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• 한국공작기계학회 1999년도 추계학술대회 논문집 - 한국공작기계학회
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• pp.339-344
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• 1999

For the Tailor Welded Blank sheet used for automobile body panel, the characteristics of fatigue strength and crack propagation behavior were studied. The thickness of specimen was machined to be same (0.9mm+0.9mm) and different (0.9mm+2.0mm). As a base test, mechanical properties around welding zone were examined. The results indicated that there were no significant decreases in mechanical properties, but hardness around welding bead is 2.3 times greater than base material. The fatigue strength was the highest when the loading direction was parallel to the welding direction, which was about 85% of tensile strength of base material. It was decreased by 8.5% when the thickness of specimen and base material was different, and it is increased by 25% when pre-strain was applied. The crack propagation rate was noticeable decreased around welding line and rapidly increased as it passed by welding line. Reviewing the shape of the crack propagation, crack width around welding line was wide around the welding zone due to retardation of crack growth, but it became narrow passing welding line due to decreased toughness.

• 한국레이저가공학회지
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• 제9권2호
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• pp.1-9
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• 2006

In this paper, we investigated the characteristics of fatigue fracture on TB(Tailored Blank) weldment by comparing the fatigue crack propagation characteristics of base metal with those of TB welded sheet used for vehicle body panels. We also investigated the influence of center crack on the fatigue characteristic of laser weld sheet of same thickness. We conducted an experiment on fatigue crack propagation on the base metal specimen of 1.2mm thickness of cold-rolled metal sheet(SPCSD) and 2.0mm thickness of hot-rolled metal sheet(SAPH440) and 1.2+2.0mm TB specimen. We also made an experiment on fatigue crack propagation on 2.0+2.0mm and 1.2+1.2mm thickness TB specimen which had center crack. The characteristics of fatigue crack growth on the base metal were different from those on 1.2+2.0mm thickness TB specimen. The fatigue crack growth rate of the TB welded specimens is slower in low stress intensity factor range $({\Delta}K)$ region and faster in high${\Delta}K$ region than that of the base metal specimens. The slant crack angle slightly influenced the crack propagation of the TB specimen of 2.0+2.0mm thinkness.

• 한국레이저가공학회지
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• 제3권2호
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• pp.53-61
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• 2000

On the laser weld production line, a slight alteration of the welding condition produces many defects. The defects are monitored in real time, in order to prevent continuous occurrence of defects, reduce the loss of material, and guarantee good quality. The measurement system is produced by using three photo-diodes for detection of the plasma and spatter signal in CO$_2$ laser welding. For high speed CO$_2$ laser welding, laser tailored welded blanks for example, on-line weld quality monitoring system was developed by using fuzzy multi-feature pattern recognition. Weld qualities were classified optimal heat input, a little low heat input, low heat input, and focus misalignment, and final weld quality were classified good and bad.

• Journal of Welding and Joining
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• 제20권2호
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• pp.95-101
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• 2002

After high power lasers are avaliable in the commercial market, the number of applications of the laser welding has been increased in manufacturing industries. Although the tailored blank laser welding of butt jointed steel sheets is well known recently in the automotive industries, the lap joint laser welding is a new technology to the automotive manufacturing people as well as the design people. But the deep penetration laser welding seems to be preferred to the partial penetration welding for the lap joint welding in the automotive manufacturers because the partial penetration is a serious deflect for the butt joint. In this study, the feasibility of partial penetration welding fur the lap joint $CO_2$ laser welding was studied fur the 1mm thick 390MPa high strength steel sheets for automotive bodies. The process window of the lap joint partial penetration welding was obtained from experiments with the gap size and the welding speed as process parameters. The partial penetration welding was found excellent on the basis of the tensile shear strength and sectional geometry. The bead width, input energy Per volume, tensile-shear strength, deformation energy and the sectional geometries after tensile-shear tests of partial penetration welded specimens are compared with those of full penetration welded specimens with a series of gaps and welding speeds.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2003년도 춘계학술대회논문집
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• pp.249-253
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• 2003

In recent years, developments of light weight vehicle are one of the most important issues in automotive makers. New materials and new processes have been studied on the point of weight saving of chassis worldwide. Associated with materials, applications of high strength steel, aluminium, magnesium are being developed. On the point of new processes, tailored welded blank and hydroforming have been applied. In this paper, focusing to both material and process, we have applied hydroforming process to the engine cradle. In addition to that, three kinds of high strength steel have been applied to the development of light weight material for hydroforming. All the studies have been carried by FEM.

• 소성∙가공
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• 제15권4호
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• pp.289-294
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• 2006

To reduce automobile parts weight, TWB(tailored welded blank) forming is widely used in panel forming. But products used TWB forming process have many defect, wrinkle, crack and springback. So study of TWB forming process character is very important. In this study one of the current problems of TWB forming was analyzed, especially for the try-out process of inner door panel without frame. A comparison was made between actual measurements and prediction of forming analysis for formability and springback. Also a new analysis die model which have additional plane on die surface was proposed to correct result of forming analysis. This proposed method overcomes the difference for TWB forming result between try-out and analysis.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2004년도 추계학술대회논문집
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• pp.132-137
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• 2004

In order to reduce automobile parts weight, TWB(Tailored welded blank) forming is widely used in the forming of car panel, such as door inner, side outer panels. In this study, one of the current problems of TWB forming was analyzed, especially for the try-out process of TWB door inner panel without frame. A comparison was made between actual panel measurements and results of forming analysis for formability and springback.